DNA Molecular Weight Calculator
Paste your DNA sequence below to estimate molecular weight (average mass). Supports A, C, G, and T only.
For unmodified strands:
MW(ssDNA) = Σ(residue masses) − 61.96.Each terminal phosphate option adds
+79.98 g/mol.
What this DNA MW calculator does
This tool estimates the molecular weight (MW) of a DNA sequence so you can quickly convert between mass and moles, verify oligo specifications, and plan downstream experiments. It works for both single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA).
The calculator is designed for practical lab use: paste sequence, pick strand type, optionally include terminal phosphate groups, and get an instant result in g/mol and kDa.
Why molecular weight matters in molecular biology
Accurate DNA molecular weight estimates help with:
- Oligo resuspension: Convert nmol into µg or mg accurately.
- PCR and qPCR setup: Prepare template and primer stocks at known molarities.
- Cloning workflows: Normalize insert and vector amounts by molar ratio, not just mass.
- NGS and library prep: Ensure consistent DNA input across samples.
- Bioconjugation chemistry: Estimate stoichiometry for modified oligos.
How the calculation works
1) Sequence composition is counted
The tool counts how many A, C, G, and T nucleotides are in your sequence. Any spaces or line breaks are ignored. Non-DNA symbols are flagged so you can fix sequence quality before calculation.
2) Average nucleotide residue masses are summed
Each base contributes a different average residue mass:
- A = 313.21 g/mol
- C = 289.18 g/mol
- G = 329.21 g/mol
- T = 304.20 g/mol
3) Strand correction and terminal chemistry are applied
For standard unmodified ssDNA, the calculator uses:
MW = Σ(residue masses) − 61.96.
If you select dsDNA, it adds the reverse-complement strand mass.
Optional terminal phosphate checkboxes add 79.98 g/mol each.
Interpreting your result
The primary result is molecular weight in g/mol. You also receive:
- kDa value for quick reporting.
- Base composition (A/C/G/T counts).
- Length in nucleotides.
- Estimated nmol per mg, useful for stock prep.
Practical tips for better accuracy
Use the exact sequence you ordered or amplified
Even a one-base difference shifts molecular weight and concentration calculations.
Keep modifications explicit
If your oligo has terminal phosphates, include them here. Other modifications (e.g., fluorescent dyes, biotin, spacers, phosphorothioates) require adding their specific mass contributions manually.
Know average vs monoisotopic mass
This page reports average molecular weight, which is typically what you want for routine concentration and stoichiometry calculations. High-resolution MS workflows may need monoisotopic values.
Example workflow
Suppose your primer sequence is ATGCGTACGTTAGC:
- Paste the sequence.
- Select ssDNA.
- Check a terminal phosphate box only if relevant.
- Click Calculate MW.
You can then convert mass to moles using:
moles = mass (g) / molecular weight (g/mol).
FAQ
Can I paste FASTA headers?
No. Remove headers and keep only nucleotide letters. The calculator accepts A/C/G/T characters.
Does this support RNA?
Not on this page. RNA uses U instead of T and different masses.
Can I calculate very long sequences?
Yes, for typical browser limits this works well. For whole chromosomes, specialized bioinformatics tools are better.